Fixed flaky MasterDetectorExpireSlaveZKSessionNewMaster test.
Fixed flaky MasterDetectorExpireSlaveZKSessionNewMaster test.

Review: https://reviews.apache.org/r/12065

verizonlabs/mesos,craimbert/mesos,neilconway/mesos,chhsia0/mesos,asamerh4/mesos,gsantovena/mesos,dforsyth/mesos,gsantovena/mesos,asamerh4/mesos,chhsia0/mesos,gsantovena/mesos,kaysoky/mesos,zmalik/mesos,neilconway/mesos,reneploetz/mesos,Gilbert88/mesos,reneploetz/mesos,jpeach/mesos,abudnik/mesos,abudnik/mesos,abudnik/mesos,neilconway/mesos,zmalik/mesos,reneploetz/mesos,reneploetz/mesos,verizonlabs/mesos,jpeach/mesos,abudnik/mesos,asamerh4/mesos,jpeach/mesos,andschwa/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,jpeach/mesos,yuquanshan/customizedMesos,reneploetz/mesos,chhsia0/mesos,andschwa/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,jpeach/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,craimbert/mesos,andschwa/mesos,craimbert/mesos,Gilbert88/mesos,gsantovena/mesos,gsantovena/mesos,asamerh4/mesos,gsantovena/mesos,asamerh4/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,chhsia0/mesos,chhsia0/mesos,zmalik/mesos,dforsyth/mesos,chhsia0/mesos,verizonlabs/mesos,dforsyth/mesos,zmalik/mesos,reneploetz/mesos,neilconway/mesos,zmalik/mesos,craimbert/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,neilconway/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,yuquanshan/customizedMesos,craimbert/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,craimbert/mesos,asamerh4/mesos,jpeach/mesos,dforsyth/mesos,kaysoky/mesos,yuquanshan/customizedMesos,andschwa/mesos,zmalik/mesos,jpeach/mesos,dforsyth/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,Gilbert88/mesos,andschwa/mesos,asamerh4/mesos,yuquanshan/customizedMesos,craimbert/mesos,dforsyth/mesos,verizonlabs/mesos,kaysoky/mesos,neilconway/mesos,zmalik/mesos